Power converter with an adaptive controller and method of operating the same

ABSTRACT

A controller for a power converter, and method of operating the same. The controller improves power converter operating efficiency by regulating an internal power converter operating characteristic depending on a value of a power converter parameter measured after a manufacturing step, or an environmental parameter, preferably employing a table with entries dependent on the parameter value. The internal operating characteristic may be an internal bus voltage, a voltage level of a drive signal for a power switch, a number of paralleled power switches selectively enabled to conduct, or a basic switching frequency of the power converter. The controller may regulate an internal operating characteristic of the power converter using a functional relationship dependent on the parameter value. The environmental parameter may be received as a signal from an external source. The parameter measured after a manufacturing step may be a parameter measured from representative power converter(s).

TECHNICAL FIELD

The present invention is directed, in general, to electronic powerconversion and, more specifically, to a power converter including acontroller adapted to improve power conversion efficiency, and a methodof operating the same.

BACKGROUND

A switch-mode power converter (also referred to as a “power converter”)is a power supply or power processing circuit that converts an inputvoltage waveform into a specified output voltage waveform. Controllersassociated with the power converters manage an operation thereof bycontrolling the conduction periods of power switches employed therein.Generally, the controllers are coupled between an input and output ofthe power converter in a feedback loop configuration.

Typically, the controller measures an internal operating characteristic(e.g., an internal bus voltage) or an output characteristic, (e.g., anoutput voltage or an output current) representing an operating conditionof the power converter, and based thereon modifies a duty cycle of apower switch or power switches of the power converter to regulate theinternal operating characteristic or the output characteristic. The dutycycle is a ratio represented by a conduction period of a power switch toa switching period thereof. Thus, if a power switch conducts for half ofthe switching period, the duty cycle for the power switch would be 0.5(or 50 percent). Additionally, as the needs for systems such as amicroprocessor powered by the power converter dynamically change (e.g.,as a computational load on the microprocessor changes), the controllershould be configured to dynamically increase or decrease the duty cycleof the power switches therein to regulate the internal or the outputcharacteristic at a desired value. In an exemplary application, thepower converters have the capability to convert an unregulated dc inputvoltage such as five volts to a lower, regulated, dc output voltage suchas 2.5 volts to power a load. In another exemplary application, thepower converters have the capability to convert an unregulated ac inputvoltage such as 120 volts to a regulated internal dc bus voltage, suchas 300 volts dc, and to further convert the regulated internal dc busvoltage into a dc output voltage such as 2.5 volts to power a load.

An important consideration for the design of a power converter and itscontroller is the efficiency (also referred to as “operatingefficiency”) in a particular application, and under particular operatingconditions. The efficiency of a power converter is the ratio of itsoutput power to its input power. The practical efficiency of a powerconverter that delivers at least half its rated output power to a loadis typically 80 to 90%. As load current is reduced, the operatingefficiency correspondingly goes down. In the limiting case wherein theload current approaches a small percentage of the maximum rated currentof the power converter, the operating efficiency approaches zero due tothe need to provide power for fixed internal loads such as thecontroller itself, for drivers for internal high-frequency powerswitches, and for inherently dissipative circuit elements such as themagnetic core of a high-frequency transformer. Power converterefficiency is accordingly dependent on an internal operatingcharacteristic of the power converter or an output characteristicthereof. Examples of an internal operating characteristic include atemperature of a component part, an internal bus voltage, the voltagelevel of a drive signal for a power switch, the number of paralleledpower switches selectively enabled to conduct, or even the basicswitching frequency of the power converter. Examples of an outputcharacteristic include a load current drawn from the power converter andan output voltage. Power converter efficiency is also dependent on aparameter that may be measured after a manufacturing step, which mayreflect a dependency of efficiency on particular parts used tomanufacture the power converter in question.

Operating efficiency is an important quality indicator for a powerconverter because of the broad impact efficiency has on equipmentreliability and size, operating expense, and corresponding effects onthe load equipment that it powers. Thus, system considerations ofachieving high operating efficiency have immediate effects on theapplicability of a particular power converter design, and the associatedprice thereof in the marketplace.

Numerous prior art attempts have been made to improve the operatingefficiency of a power converter. Most attempts have focused on selectionof proper components to provide the maximum operating efficiency foraverage operating conditions at a chosen operating point, such as a loadcurrent at three quarters of a maximum rated value, the environmentaltemperature at a typical expected value, and for a typical mix of actualcomponents employed to manufacture a particular power converter unit.Recognizing the wide range of possible values for any of theseparameters, there is substantial remaining opportunity to improve theefficiency of a power converter for a particular operating condition.

An example of the prior art to provide high power converter efficiencyat a particular operating condition is provided in U.S. Pat. No.6,351,396, entitled “Method and Apparatus for Dynamically AlteringOperation of a Converter Device to Improve Conversion Efficiency,” toJacobs, issued Feb. 26, 2002, which is incorporated herein by reference.Jacobs is directed to a search process that varies parameters accessibleto the controller during power converter operation, such as a timingdelay between conduction intervals of the power switches, observes theresulting effect on the duty cycle. The duty cycle is employed as anindicator of operating efficiency, and parameters accessible to thecontroller are adjusted to produce an extremum in the duty cycle for aparticular operating condition, thereby increasing the operatingefficiency of the power converter. While Jacobs performs efficiencyoptimization under actual operating conditions, the referencenonetheless fails to consider constraints of the actual application(such as a requirements document or operating specification document) orthe environment during execution of the process of efficiencyoptimization, or a signal from an external source to limit or alter theoptimization process. For example, no attempt is made to measure aparameter of a particular power converter unit after a manufacturingstep (or to measure a parameter of a representative power converterunit), or to control, program, or otherwise alter a response of thecontroller to reflect such measurement, such as by controlling aninternal operating characteristic or an output characteristic.

Another attempt to adaptively operate a power converter to improveefficiency is described in U.S. Pat. No. 5,742,491, entitled “PowerConverter Adaptively Driven,” to Bowman, et al. (“Bowman”), issued Apr.21, 1998, which is incorporated herein by reference. Bowman is directedto a drive circuit for a power converter wherein the timing ofconduction intervals for the power switches is programmed to increasethe efficiency of the power converter while keeping stresses onindividual components within acceptable limits. A predetermined delaybetween drive waveforms supplied to the power switches and to thesynchronous rectifiers of the power converter is altered with apredetermined program that is a function of an operating condition ofthe power converter to allow the power converter to operate efficientlyin an anticipated operating environment and with anticipated componentrealizations. A design objective is to desensitize the operatingefficiency to an expected range of changes in the operating environmentand with an anticipated range of component realizations, which resultsin a compromise in a static program to optimize efficiency that mightotherwise be achievable with the design of an improved controller not solimited. Bowman relies on a limited set of a priori conditions, and doesnot adjust controller parameters in response to a measured powerconverter parameter for the particular power converter unit after amanufacturing step, or to a measured parameter of a representative powerconverter unit, or in response to a signal from an external sourcerepresenting an environmental parameter.

A further attempt to optimize power conversion efficiency is describedin U.S. Pat. No. 5,734,564, entitled “High-Efficiency Switching PowerConverter,” to Brkovic, issued Mar. 31, 1998, which is incorporatedherein by reference. Brkovic describes measuring an internal operatingcharacteristic of a power train of the power converter (i.e., a voltageacross a power switch) and adjusting a timing of a duty cycle for thepower switch in response to the measured power switch voltage to improvepower conversion efficiency. Brkovic provides a preconditioned responseto a measured parameter of the particular power converter unit after amanufacturing step. Brkovic does not consider adapting or constrainingthe response to a signal from an external source representing anenvironmental parameter.

It is well known in the art to couple an input control signal to a powerconverter to control the setpoint of an output characteristic thereof.For example, the output voltage of a power converter adapted to supplypower to a microprocessor load (wherein the operating voltage thereof isnot known at the time of manufacture or that is changed during normaloperation such as when a microprocessor enters a sleep mode) can bestatically or dynamically altered by an input control signal. However,this control mechanism merely changes a setpoint for an outputcharacteristic of the power converter, and is not adapted to optimizethe efficiency of the power converter at the signaled setpoint.

Thus, attempts have been made in the prior art to configure controllersto statically optimize power conversion efficiency of a power train. Thestatic responses have included varying an internal operatingcharacteristic of the power converter with a fixed program to alter aninternal operating characteristic of the power converter in response toa measured characteristic such as a load current to improve powerconversion efficiency. The aforementioned attempts to improve efficiencyhave been facilitated by inclusion of programmable digital devices suchas microprocessors, digital signal processors, application specificintegrated circuits, and field-programmable gate arrays in thecontroller. Nonetheless, the responses of a controller have not includedconsideration of a measured parameter after a manufacturing step for theparticular power converter unit that is being controlled such as ameasurement of an actual delay of a particular power switch or aninternal circuit after completion of a stage of manufacture.

Considering limitations as described above, a controller for a powerconverter is presently not available for the more severe applicationsthat lie ahead that depend on achieving higher operating efficiency fora particular operating characteristic constrained or controlled by anenvironmental parameter. In addition, a controller for a power converteris presently not available that responds to a parameter measured after amanufacturing step for the particular power converter unit, or to aparameter measured after a manufacturing step on a representative powerconverter unit, or on power converter units in a representative run, toimprove the operating efficiency thereof.

Accordingly, what is needed in the art is a controller for a powerconverter that adaptively improves power conversion efficiency of apower converter in response to a measured parameter of the powerconverter after a manufacturing step, or to a parameter measured on arepresentative power converter unit, and includes consideration ofoperating conditions and a signal from an external source representingan environmental parameter. In accordance therewith, a controller forpower converter is provided that adaptively improves power conversionefficiency, including the considerations as provided herein.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by advantageous embodimentsof the present invention which include a controller for a powerconverter and a method of forming the same. In one embodiment, thecontroller includes a power switch configured to conduct for a dutycycle and provide a regulated output characteristic at an outputthereof. In a preferred embodiment, the controller is configured toprovide a signal to control the duty cycle of the power switch as afunction of the output characteristic by regulating an internaloperating characteristic of the power converter to improve powerconverter operating efficiency. The internal operating characteristic isregulated depending on a value of one of a parameter of the powerconverter measured after a manufacturing step and an environmentalparameter for the power converter.

In a preferred embodiment, the controller regulates the internaloperating characteristic of the power converter in accordance with atable with entries dependent on a value of one of the parameter of thepower converter measured after the manufacturing step and theenvironmental parameter of the power converter. In a further preferredembodiment, the internal operating characteristic is one of an internalbus voltage, a voltage level of a drive signal for the power switch, anumber of paralleled power switches selectively enabled to conduct, atemperature of a component part, a timing relationship between two ormore power switches, or a basic switching frequency of the powerconverter. In a further preferred embodiment, the controller regulatesan internal operating characteristic of the power converter inaccordance with a functional relationship dependent on a value of one ofthe parameter of the power converter measured after a manufacturing stepand an environmental parameter for the power converter, which may be anenvironmental parameter received as a signal from an external source.

In a further preferred embodiment, the internal operating characteristicis regulated by the controller on a time scale substantially differentfrom a time scale for controlling the duty cycle of the power switch. Ina further preferred embodiment, the parameter of the power convertermeasured after a manufacturing step is measured automatically in a testfixture. In a further preferred embodiment, the parameter of the powerconverter measured after a manufacturing step is a parameter measuredfrom a representative power converter unit, or from power converterunits produced during a run of representative power converter units. Ina further preferred embodiment, the internal operating characteristic ofthe power converter is controlled in a step-by-step manner during anefficiency optimization process. In a further preferred embodiment, theinternal operating characteristic of the power converter is controlledin a step-by-step manner during an efficiency optimization process on atime scale substantially different from a time scale for controlling theduty cycle of the power converter.

In another aspect and for use with a power converter couplable to asource of electrical power adapted to provide an input voltage thereto,the power converter includes a power switch configured to conduct for aduty cycle and provide a regulated output characteristic at an outputthereof. The present invention provides a method of operating acontroller for the power converter to improve power converter operatingefficiency so that an internal operating characteristic is regulateddepending on a value of one of a parameter of the power convertermeasured after a manufacturing step and an environmental parameter forthe power converter. In a preferred embodiment, the controller providesa signal to control the duty cycle of the power switch as a function ofthe output characteristic by regulating an internal operatingcharacteristic of the power converter. In a preferred embodiment, themethod includes regulating the internal operating characteristic of thepower converter in accordance with a table with entries dependent on avalue of one of the parameter of the power converter measured after themanufacturing step and the environmental parameter of the powerconverter. In a further preferred embodiment, the method includesregulating the internal operating characteristic dependent on one of aninternal bus voltage, a voltage level of a drive signal for a powerswitch, a number of paralleled power switches selectively enabled toconduct, a temperature of a component part, a timing relationshipbetween two or more power switches, or a basic switching frequency ofthe power converter. In a further preferred embodiment, the methodincludes regulating the internal operating characteristic of the powerconverter in accordance with a functional relationship dependent on avalue of one of the parameter of the power converter measured after amanufacturing step and an environmental parameter for the powerconverter, which may be an environmental parameter received as a signalfrom an external source.

In a further preferred embodiment, the method includes regulating theinternal operating characteristic on a time scale substantiallydifferent from a time scale for controlling the duty cycle of the powerswitch. In a further preferred embodiment, the method includes measuringa parameter of the power converter automatically in a test fixture aftera manufacturing step. In a further preferred embodiment, the methodincludes measuring a parameter from a representative power converterunit, or from power converter units produced during a run ofrepresentative power converter units, to control the parameter of thepower converter. In a further preferred embodiment, the method includesadjusting the internal operating characteristic of the power converterto improve an operating efficiency in a step-by-step manner during anefficiency optimization process. In a further preferred embodiment, themethod includes adjusting the internal operating characteristic of thepower converter on a time scale substantially different from a timescale for controlling the duty cycle of the power converter.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a power converter controlled by aconventional controller;

FIG. 2 illustrates a schematic diagram of an exemplary power train of abuck power converter;

FIG. 3 illustrates a schematic diagram of an embodiment of a powerconverter including a controller constructed according to the principlesof the present invention;

FIG. 4 illustrates exemplary waveform diagrams to control the conductionintervals of selected power switches of the power converter of FIG. 3with an intervening delay therebetween;

FIGS. 5A and 5B illustrate diagrams of exemplary multidimensional tablesfor the time delays for the control signals to control the conductionintervals of synchronous rectifier switches in accordance with arepresentative operating parameter of the power converter of FIG. 3;

FIG. 6 illustrates an embodiment of a functional representation toimprove power conversion efficiency constructed according to theprinciples of the present invention;

FIG. 7 illustrates a block diagram of an embodiment of a power converterconstructed according to the principles of the present invention;

FIG. 8 illustrates a block diagram of an embodiment of a power converterconstructed according to the principles of the present invention;

FIGS. 9A to 9F illustrate the dependence of power converter efficiencyon various operating parameters and the operating environment inaccordance with the principles of the present invention; and

FIG. 10 illustrates an ac input voltage waveform including an exemplaryinput line voltage dropout transient, showing time histories of possibleinternal bus voltages in accordance with the principles of the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to exemplaryembodiments in a specific context, namely, a controller for a powerconverter and, more particularly, a controller for a power converterthat regulates an output characteristic of the power converter at anoutput thereof that adaptively controls an internal operatingcharacteristic to increase power conversion efficiency in response to aparameter of the power converter measured after a manufacturing stepand/or an environmental parameter of the power converter. The parametersmentioned above are typically measured after the power converter(s) areimplemented and/or after a signal is received from an external sourcerepresenting an environmental parameter. Examples of an environmentalparameter include, without limitation, a signal indicating the existenceof a paralleled power converter, the operational state of the paralleledpower converter, that the powered system is operating from a backuppower source, a request for a particular load voltage, an indicationthat a particular portion of the load has failed, or has been disabled,or is operating at a reduced power level.

Additionally, the controller for a power converter according to theprinciples of the present invention can control, alter, or otherwiseconstrain an internal operating characteristic (such as a gate drivevoltage level, a switching frequency, an internal voltage or current,etc.) or an output characteristic (such as a regulated voltage setpointof the power converter) to improve an efficiency thereof in response toa signal from an external source representing an environmental parameter(such as the existence of a parallel-coupled power converter powering acommon load). For example, the internal dc bus voltage of a powerconverter might be adaptively reduced to improve the power conversionefficiency of a front-end boost power converter, but such voltagereduction would directly affect the holdover capability of the powerconverter during periods of loss of ac input voltage (often referred toas line dropout), which might be a required internal operatingcharacteristic. Holdover capability is generally inversely proportionalto the load on the power converter and would depend on the presence andoperational state of a paralleled power converter. The data from anexternal source representing an environmental parameter can be employedby an adaptive controller, for example, to reduce the internal dc busvoltage to a particular level above a lower voltage limit dependent onthe measured power converter load and the external data, and therebyimprove operating efficiency in view of an internal characteristic or anoutput characteristic, but constrained by the signal from the externalsource.

Referring initially to FIG. 1, illustrated is a block diagram of a powerconverter controlled by a conventional controller 110. The powerconverter includes a power train 105 coupled to a source of electricalpower (represented by a battery, but may be other sources of power, suchas ac power) for providing an input voltage V_(in) for the powerconverter. The power converter also includes a controller 110, andprovides power to a system (not shown) such as a microprocessor coupledto an output thereof. The power train 105 may employ a buck topology asillustrated and described with respect to FIG. 2 below.

The power train 105 receives an input voltage V_(in) at an input thereofand provides a regulated output characteristic (e.g., an output voltageV_(out)) to power a microprocessor or other load coupled to an output ofthe power converter. The controller 110 is typically coupled to avoltage reference representing a desired characteristic such as adesired system voltage from an internal or external source associatedwith the microprocessor, and to the output voltage V_(out) of the powerconverter. In accordance with the aforementioned characteristics, thecontroller 110 provides a signal to control a duty cycle and a frequencyof at least one power switch of the power train 105 to regulate theoutput voltage V_(out) or another characteristic thereof. Thus, thecontroller 110 for the power train 105 of a power converter,particularly a switch-mode power converter, generally measures aninternal operating characteristic or an output characteristic of thepower converter and controls a duty cycle of a power switch therein inresponse to the measured characteristic to regulate the internaloperating characteristic or the output characteristic thereof.

A driver (not shown) may be interposed between the controller 110 andthe power train 105 to provide a drive signal(s) for the powerswitch(es) with sufficient amplitude and with waveform characteristicsto efficiently enable or disable conductivity of the power switch(es).In accordance with the aforementioned characteristics, a drive signal isprovided by a driver to control a duty cycle and a frequency of one ormore power switches of the power converter, preferably to regulate theoutput voltage V_(out) thereof. For a P-channel metal-oxidesemiconductor power switch, a gate drive signal is typically drivennegative (with respect to the source terminal) to turn on the powerswitch, and for an N-channel metal-oxide semiconductor power switch, agate drive signal is typically driven positive (with respect to thesource terminal) to turn on the power switch. A driver may employtechniques to provide sufficient signal delays to prevent shoot-throughcurrents when controlling multiple power switches in a power converter.

Turning now to FIG. 2, illustrated is a schematic diagram of anexemplary power train of a buck power converter. The power train of thepower converter receives an input voltage V_(in) (e.g., an unregulatedinput voltage) from a source of electrical power (represented by abattery) at an input thereof and provides a regulated output voltageV_(out) to power, for instance, a microprocessor at an output of thepower converter. In keeping with the principles of a buck topology, theoutput voltage V_(out) is generally less than the input voltage V_(in)such that a switching operation of the power converter can regulate theoutput voltage V_(out). A main power switch Q_(main) is enabled toconduct by a gate drive signal D for a primary interval and couples theinput voltage V_(in) to an output filter inductor L_(out). During theprimary interval, an inductor current I_(out) flowing through the outputfilter inductor L_(out) increases as a current flows from the input tothe output of the power train. An ac component of the inductor currentI_(Lout) is filtered by an output capacitor C_(out).

During a complementary interval, the main power switch Q_(main) istransitioned to a non-conducting state and an auxiliary power switchQ_(aux) is enabled to conduct by a complementary gate drive signal 1-D.The auxiliary power switch Q_(aux) provides a path to maintain acontinuity of the inductor current I_(Lout) flowing through the outputfilter inductor L_(out). During the complementary interval 1-D, theinductor current I_(Lout) through the output filter inductor L_(out)decreases. In general, the duty cycle of the main and auxiliary powerswitches Q_(main), Q_(aux) may be adjusted to maintain a regulation ofthe output voltage V_(out) of the power converter. Those skilled in theart understand that the conduction periods for the main and auxiliarypower switches Q_(main), Q_(aux) may be separated by a small timeinterval to avoid cross conduction current therebetween and beneficiallyto reduce the switching losses associated with the power converter.Thus, the power train of a switch-mode power converter generallyincludes a plurality of power switches coupled to reactive circuitelements to provide the power conversion function therefore.

Turning now to FIG. 3, illustrated is a schematic diagram of anembodiment of a power converter including a controller 311 constructedaccording to the principles of the present invention. The powerconverter includes two exemplary power stages, namely, a first powerstage 310 (e.g., a boost power stage possibly employed to perform powerfactor correction) and a second power stage (e.g., an isolating dc-to-dcpower stage) 320. The input power source 301 to the first power stage310 is an ac power source, which is coupled to a diode bridge rectifier303. The first power stage 310, controlled by controller 311, producesan internal regulated voltage V_(bus) across a capacitor C₁, whichprovides the input voltage to the second power stage 320. The firstpower stage 310 includes boost power switch Q_(boost) and diode D₁,which alternately conduct to transfer charge from the input power source301 through an inductor L₁ to the capacitor C₁. The controller 311senses the rectified input voltage V_(in) and the internal bus voltageV_(bus) to control a duty cycle of the boost power switch Q_(boost), toregulate the bus voltage V_(bus) and to control the power factor ofpower drawn from the input power source 301.

The second power stage 320 includes isolation transformer TR and a powerswitch Q_(pri) in series with the primary winding thereof. Synchronousrectifier switches Q_(fwd), Q_(fly) are power switches coupled in seriesacross a secondary winding of the isolation transformer TR to rectifythe voltage therefrom, which winding voltage is coupled to an outputfilter including an output inductor L_(out) and an output capacitorC_(out). The controller 311 provides control signals (e.g., gate controlsignals) V_(Gfwd), V_(Gfly) to control the synchronous rectifierswitches Q_(fwd), Q_(fly), respectively. A brief time delay ΔT betweenconduction intervals of the synchronous rectifier switches Q_(fwd),Q_(fly) is provided by the controller 311 to prevent cross conductiontherebetween. In a preferred embodiment, the controller 311 selects thetime delay ΔT dependent on operating conditions of the power converteras described hereinbelow to provide improved power conversionefficiency.

Turning now to FIG. 4, illustrated are exemplary waveform diagrams tocontrol the conduction intervals of selected power switches of the powerconverter of FIG. 3 with an intervening delay therebetween. Morespecifically, FIG. 4 illustrates an example of a time delay ΔT betweenthe gate control signals V_(Gfwd), V_(Gfly) to control the conductionintervals of the synchronous rectifier switches Q_(fwd), Q_(fly),respectively.

Returning now to the description of the power converter of FIG. 3, theoutput filter attenuates ac components present across the secondarywinding of transformer TR to provide a substantially constant dc outputvoltage V_(out). The output voltage V_(out), as well as the load currentI_(load) sensed by a sensor (e.g., a current sensing circuit element315), is sensed by controller 311. Various circuit elements capable ofsensing a load current, including a low resistance current-sensingresistor or a current-sensing transformer, are well known in the art andwill not be described herein. Although the controller 311 is showndirectly coupled to both sides of the isolation transformer TR, circuitelements to provide the necessary level of isolation for a controller311 for a particular application are well known in the art and will notbe described herein. A thermistor (or other suitable thermal sensor) 313provides a temperature measurement to the controller 311 at a selectedpoint(s) in or about the power converter. Typical points for temperaturesensing include a location adjacent to a power switch or to an isolationtransformer, and may include the ambient temperature outside the powerconverter itself. Although one thermistor 313 is illustrated in FIG. 3,a plurality of thermistors may also be included within the scope of theinvention to provide multiple temperature measurements to the controller311. The detailed operation and characteristics of the first and secondpower stages 310, 320 are well known in the art, and will not be furtherdescribed herein.

The controller 311 in the exemplary power converter illustrated in FIG.3 senses and is responsive to the rectified input voltage V_(in), theinternal bus voltage V_(bus), the power converter output voltageV_(out), the load current I_(load), as well as a signal “Temp”representing a temperature using the thermistor 313. In addition, thecontroller 311 senses and is responsive to an external signal V_(ext)indicating an environmental parameter from an external source such as aserver powered by the power converter, and to a setup signal V_(setup)that may provide the result of a parameter measured in a test fixtureafter a manufacturing step to set or otherwise tailor parameters for theoperation of the controller 311. In a preferred embodiment, thecontroller 311 includes digital processing capability at leastcomparable to that of a low-end microprocessor (or other digitalimplementations, such as a microcontroller, digital signal processor, afield-programmable gate array, or complex programmable logic device),and is operative to adaptively improve (e.g., optimize) the efficiencyof the power converter from a variety of data and signal sources.

The controller 311 is configured to augment the operating efficiency ofthe power converter in response to a sensed or signaled internaloperating characteristic and/or an output characteristic, a powerconverter parameter measured after a manufacturing step, and a signalfrom an external source representing an environmental parameter obtainedfrom an external source such as a signal from a server being powered.Exemplary environmental parameters obtained from an external source,which reflect how the power converter is being used in an application,include a signal indicating parallel operation with a second powerconverter, an indication that a paralleled power converter has failed,an indication that the power converter is supporting a criticalapplication requiring a modified trade-off between power conversionefficiency and reliability, and an indication that the system isoperating from a back-up power source, and may signal, for example, alower limit for a dc bus voltage, reflecting a modified need for powerconverter holdover to accommodate altered statistics for a transientpower outage condition.

The controller 311 may include a multidimensional table or otherfunctional representation of a value to control an internal operatingcharacteristic or an output characteristic of the power converter.Multidimensional inputs to such a table or other functionalrepresentation include signals representing an internal operatingcharacteristic, an output operating characteristic, a power converterparameter measured after a manufacturing stage, a parameter measured ona representative power converter unit, and/or a signal representing anenvironmental parameter. There are references utilizing lookup tablesand other multidimensional functional representations directed toautomotive engine map and lookup table systems such as U.S. Pat. No.5,925,088, entitled “Air-fuel Ratio Detecting Device and Method,” toNasu, issued Jul. 20, 1999, U.S. Pat. No. 7,076,360, entitled“Auto-Ignition Timing Control and Calibration Method, to Ma, issued Jul.11, 2006, and U.S. Pat. No. 6,539,299, entitled “Apparatus and Methodfor Calibrating an Engine Management System,” to Chatfield, et al.,issued Mar. 25, 2003, which are incorporated herein by reference.

Turning now to FIGS. 5A and 5B, illustrated are diagrams of exemplarymultidimensional tables for the time delays ΔT (in nanoseconds) for thegate control signals V_(Gfwd), V_(Gfly) to control the conductionintervals of the synchronous rectifier switches Q_(fwd), Q_(fly),respectively, in accordance with a representative operating parameter ofthe power converter of FIG. 3. More specifically, FIG. 5A demonstratesthe time delay ΔT with the input voltage V_(in) being below about 48volts and FIG. 5B demonstrates the time delay ΔT with the input voltageV_(in) being above about 48 volts. The tables, listing delay innanoseconds between opening a first power switch and closing a secondpower switch (e.g., the synchronous rectifier switches Q_(fwd), Q_(fly))is accessed along a row with suitably quantized load current I_(load),and along a column with suitably quantized temperature. The entries inthe table are obtained by experimentally varying switch delay in a testset after manufacture of the power converter, and observing the effectof various delays on power conversion efficiency. The tables reflect arange of different values of input voltage V_(in) measured for theparticular power converter unit after a manufacturing stage. Of course,tables can be constructed with additional dimensions, accommodatingadditional parameters such as an output voltage V_(out), and internalbus voltage V_(bus), an input signal from an external source indicatingan environmental parameter, etc., and finer levels of granularity.Various methods of interpolation between entries in the tables are wellknown in the art, and will not be described in the interest of brevity.

Such multidimensional tables can be used, for example, to control theswitching frequency of a power converter. Switching frequency in theprior art is generally set as a design parameter, and is selected andfixed during a stage of design. The selected switching frequency isgenerally the result of a trade-off that considers, for example, theloss characteristics of the core material of the isolation transformerwhich depend on, without limitation, transformer core temperature, theprimary-to-secondary turns ratio of the transformer, the expectedthermal environment of the application, the heat transfercharacteristics of the resulting power converter design, and theparticular batch of core material from which the magnetic core thereofwas formed. The resulting core loss for a particular power converterunit can also be substantially dependent on core characteristics such asa flux gap and core area of the particular core that was installed, allof which are substantially unknown before the power converter ismanufactured.

In addition, the selected switching frequency is a result ofconsideration of other frequency dependent losses within the powerconverter. For example, gate drive losses are generally proportional toswitching frequency and depend on the particular manufacturing run ofpower switches employed therein. Thus, altering the switching frequencyfor a particular application using a table constructed according to theprinciples of the present invention, considering manufacturing data,actual load current, and other measured or sensed variables can resultin improved power conversion efficiency within a predetermined set ofoperating constraints that may be signaled from an external source. Atest set can be readily constructed, as is well known in the art, tovary switching frequency and observe the effect on power conversionefficiency. Entries are then made in the table to represent preferableswitching frequencies. Static efficiency optimization approaches of theprior art that use a predetermined curve or other fixed approach do notadvantageously achieve the benefits of improved efficiency with greaterflexibility to respond to additional data as described herein.

Turning now to FIG. 6, illustrated is an embodiment of a functionalrepresentation to improve power conversion efficiency constructedaccording to the principles of the present invention by determining acontrollable parameter such as an internal bus voltage setpoint V_(bus)_(—) ^(setpoint) of the power converter. An exemplary function isrepresented dependent on load current I_(load), operating temperature,data acquired after a manufacturing step, and data from an externalsource. The exemplary functional dependence illustrated in FIG. 6 forthe internal bus voltage setpoint V_(bus) _(—) ^(setpoint) for aninternal bus voltage is:

V _(bus) _(—) ^(setpoint) =380+0.1·I _(load)−0.2·Temp+V _(setup)+10·V_(ext),

where “I_(load)” represents a sensed power converter load current,“Temp” represents a sensed temperature using a thermistor or othertemperature sensing element for a location in or about the powerconverter, “V_(setup)” represents a correction constant obtained from atest set after a manufacturing step, and “V_(ext)” represents a signalfrom an external source that might assume the values 0 and 1 to indicatethe presence or absence of a paralleled power converter (see, e.g., FIG.3 and the related description therefore). A constant “380” is a nominalnumber to describe the internal bus voltage setpoint V_(bus) _(—)^(setpoint). Other functional relationships including combinations ofcurve fits or other algorithmic relationships can be used within thebroad scope of the present invention to meet the needs of a particularapplication. The controller 311 illustrated in FIG. 3 may use theinternal bus voltage setpoint V_(bus) _(—) ^(setpoint) as a referencevoltage to control the internal bus voltage V_(bus) illustrated anddescribed with reference to FIG. 3.

The use of tables, functional relationships, and curve fits to controlan operating parameter for a controller of a power converter,constructed according to the principles of the present invention, canadvantageously use the extensive data ordinarily acquired by testfixtures at various stages of the manufacturing process. Such testfixtures are generally configured to sweep a broad range of operatingconditions for a particular power converter unit, or from arepresentative power converter unit, or from power converter unitsproduced during a run of representative power converter units, and caneven operate the power converter unit over a range of temperatures andfor an extended period of time (e.g., during “burn in”). A test fixturecan be arranged to operate a power converter over a range of trialvalues for a controllable parameter, and select a value that provides apreferable operating efficiency for the particular power converter undertest. Thus, the efficiency program for a particular power converter canbe tailored to represent the particular characteristics of theindividual components from which the power converter unit is built. In apreferred arrangement, the test fixture is programmed to automaticallysearch for the best value for the controllable parameter.

It is recognized that the timescale for the response of a controller todifferent internal and external stimuli can preferably be different. Forexample, the voltage level of an internal bus, which generally dependson charging and discharging a capacitor, might be practically changedover a period of hundreds of milliseconds, or even seconds, whereas theswitching frequency of a power conversion stage or the timing delaybetween power switch conduction intervals can be readily changed on amuch faster time scale, ultimately on a cycle-by-cycle basis. It mayeven be inappropriate to substantially change operating parameters suchas an internal bus voltage level over intervals of time shorter thanseveral seconds. Some internal operating characteristics or parameterswould inherently change or would be inherently varied over a relativelylong period, such as the input current of an ac front end, compared toother time scales within a power converter, and require a period of timeto sense or alter an average or peak value. Such internal parameters maybe monitored over a longer time interval before the controller respondsto a change in an internal operating characteristic or an outputcharacteristic to augment power conversion efficiency. Thus, forexample, a controller may control an internal operating characteristicof a power converter in a step-by-step manner during an efficiencyoptimization process on a time scale substantially different from a timescale for controlling the duty cycle of the power converter. A parametercan be controlled on a slow timescale by using a digital representationof a low pass filter to retard changes in a parameter. An exemplaryequation representing a low pass filter implemented over discrete timesteps is:

V _(bus,n)=(1−τ)·V _(bus,n-1) +τ·V _(bus,desired)

where “V_(bus, n)” represents a filtered bus reference voltage at timestep “n” to control an internal bus voltage on a slow time scale, “τ”represents a parameter that sets the time scale for the filteringprocess, “V_(bus, n-1)” represents the filtered bus reference voltage atthe previous time step “n−1,” and “V_(bus,desired)” represents adesired, optimized bus voltage produced by a functional relationship ora table as described hereinabove.

In a related embodiment, a controller for a power converter may optimizethe operating efficiency (or other desirable parameter) of the powerconverter in response to a sensed or signaled internal operatingcharacteristic and/or an output characteristic, using parametersmeasured on a representative power converter. For example, amultidimensional table or other functional representation of a value tocontrol an internal operating characteristic or an output characteristicof the power converter could be derived from testing one or morerepresentative power converters, as opposed to testing the actual powerconverter to be controlled. Multidimensional inputs to such a table orother functional representation may include, without limitation, signalsrepresenting an internal operating characteristic, an output operatingcharacteristic, a power converter parameter measured during a test orcharacterization phase, and/or a signal representing an environmentalparameter.

During a typical power converter product development process, a productdesign may proceed through several stages, for example, prototyping,pilot (or small volume) production, characterization and/orqualification testing, safety agency and electromagnetic interference(“EMI”) compliance testing, highly accelerated life testing, highlyaccelerated stress screening, and final release to production. Duringthe characterization and/or qualification testing phase, one or morerepresentative power supplies may be subjected to extensive testing toensure compliance with the end specification. This testing may beautomated by one or more racks of automated test equipment, enablingpossibly many thousands of individual tests to be performed.

During an exemplary characterization testing stage, a representativepower converter may be extensively tested over a wide variety ofoperating conditions. Such a characterization test may measure andcollect thousands, or tens of thousands of individual data points. Thesedata may then be compiled into one or more multidimensional tables orother functional representation(s) and used by the control circuit toadjust an internal operating characteristic or an output characteristicof the power converter in order to operate the power converter at ornear an optimal efficiency for a given set of conditions, while stillenabling the power converter to meet its required specification.

Turning now to FIG. 7, illustrated is a block diagram of an embodimentof a power converter constructed according to the principles of thepresent invention. In the exemplary embodiment illustrated in FIG. 7, ablock diagram of an ac input, power factor correction, dc output powerconverter is depicted. The power converter operates from a power sourceproviding 85 to 264 V ac input, and provides outputs of +12V and 3.3VSB(a standby voltage). The power converter also provides output signalsPS_ON and POK indicating, respectively, that the power converter isturned on and power is “OK,” as well as other “communications” signalstypically provided between a power converter and a host system. It isreadily understood by those skilled in the art that there are many waysto design an ac-to-dc power converter, and correspondingly there aremany possible block diagrams that could suitably depict an exemplarypower converter. It is also understood that the spirit and scope of thepresent invention is not limited to ac-to-dc power converters, but mayencompass any type of power converter, including ac and/or dc input, aswell as ac and/or dc output. Multiple input and/or multiple output powerconverters are also within the spirit and scope of the presentinvention.

FIG. 7 illustrates many of the constituent blocks of a power converterthat may be controlled, as well as many of the internal nodes that maybe measured and/or controlled to improve operating efficiency. Forexample, a switching frequency of the boost field-effect transistors(“FETs”), and/or the bridge, may be adjusted based on operatingconditions to improve efficiency. Additionally, the voltage on the 400Vbus may be adjusted, or the timing between bridge switches and asynchronous rectifier device (“sync rect”) may be adjusted.

Turning now to FIG. 8, illustrated is a block diagram of an embodimentof a power converter (e.g., an ac-to-dc power converter) constructedaccording to the principles of the present invention and demonstratingin more detail possible control and alarm circuit connections. Thesecontrol and alarm circuits may be realized using dedicatedfirmware-driven microcontrollers, digital control integrated circuits,application specific integrated circuits, field-programmable gatearrays, or any suitable electronic circuitry. The power factorcorrection (“PFC”) control and primary alarm blocks (part of the primarycontrol) of FIG. 8 illustrate some of the many internal nodes andcircuits that may be measured and controlled. For example, the primarycontroller may monitor the input line voltage, frequency, and current,etc. It may also monitor the PFC output bus voltage (shown here as the400V bus, although the bus voltage may be controlled to other voltagelevels). The primary controller may control the PFC boost power switchesusing a variety of control techniques, including fixed and variablefrequency, continuous current mode, discontinuous current mode, orcritically continuous inductor current, to name but a few. The powerconverter could also employ additional components to achieve, forexample, soft switching, with the controller capable of measuring and/oraltering operating parameters affecting these additional components. Theprimary controller may also be capable of communicating with a secondarycontroller, and this communication may be bidirectional.

The secondary controller, including the pulse-width modulation (“PWM”)control and alarm circuits, may monitor and control the parameters shownin FIG. 8, as well as others not shown. The secondary control can thusbe used to control, among other things, switching frequency, operatingmode, output voltage, timing relationships, etc. The secondary controlmay advantageously also enable or disable the operation of individualpower switches (or banks of power switches) to improve power conversionefficiency. The illustrated embodiment of FIG. 8 also shows a means ofcommunication allowing the power converter to communicate with a widevariety of devices, including but not limited to, a host processor, oneor more pieces of automated test equipment, or another power converter.The communication protocol in the illustrated embodiment is a wired I²Cbus, but could be realized with any suitable communication means orprotocol, including wired and wireless, optical, radio frequency, etc.Additionally, the communications means need not be restricted to thesecondary side, but may be located on the primary side, or be on bothprimary and secondary sides.

Turning now to FIGS. 9A thru 9F, illustrated are examples of how powerconverter efficiency can vary as a function of operating conditions andoperating environment in accordance with the principles of the presentinvention. These curves in FIGS. 9A thru 9F are merely illustrative of afew of the parameters or environmental conditions affecting powerconversion efficiency, and are by no means meant to be exhaustive. Inaddition, the curve shapes and variations illustrated in FIGS. 9A thru9F are meant for illustrative purposes only. The efficiency of differentpower converter designs may vary in a manner different from theexemplary curves.

In FIG. 9A, the efficiency of the PFC section is illustrated as afunction of both output power and input line voltage. In FIGS. 9A thru9F, the arrows point in the direction of an increasing parameter. InFIG. 9B, the efficiency of the dc-to-dc section is illustrated as afunction of both output power and bus voltage. In FIG. 9C, theefficiency of the PFC section is illustrated as a function of bothoutput power and switching frequency at a single line voltage. A familyof such curves could be generated at different ac line voltages. In FIG.9D, the efficiency of the dc-to-dc section is illustrated as a functionof both output power and switching frequency at a single bus voltage. Afamily of such curves could be generated at different dc bus voltages.In FIG. 9E, the efficiency of the power converter (PFC plus dc-to-dcsections) is illustrated as a function of both output power and busvoltage at a single line voltage. A family of such curves could begenerated at different ac line voltages. Lastly, in FIG. 9F, theefficiency of the power converter (PFC plus dc-to-dc sections) isillustrated as a function of both output power and the timing delaybetween the bridge and synchronous rectifier switches, at a single linevoltage. A family of such curves could be generated at different ac linevoltages. Clearly, many other relationships could be measured for theireffect on power converter efficiency, including but not limited to,temperature (internal and/or external), altitude, fan speed, number ofpower switching devices enabled, etc.

The number of different relationships that could be measured and datapoints collected is limited only by the ingenuity of the test engineer,time, and data memory resources. Over many such projects, an engineermay learn that certain relationship data has more of an impact onefficiency than others, and may learn how to intelligently limit thenumber of tests performed and data points collected to only thoserelationships having the greatest effect on efficiency.

Once the data is collected on one or more representative power converterunits, multidimensional data table(s) or other functionalrepresentation(s) may be stored into the power converter's internalcontrol memory for use during operation. This stored data couldcomprise, for example, a look-up table, an algorithm, or any othersuitable method of converting test data into an actionable controlparameter. For example, assume an exemplary power converter constructedaccording to the principles of the present invention were operating in aserver, perhaps in a data center. The exemplary power converter maymeasure one or more environmental and operating conditions. The powerconverter may determine that it is operating at 20% load, at 120V acinput at 59.9 Hz, with an inlet ambient temperature of 35° C. (otherparameters could also be measured). The primary and/or secondarycontroller(s) may then access a stored look-up table that specifies, forexample, the proper switching frequency, bus voltage operatingconditions, and switch timing relationships in order to improve oroptimize efficiency. The controllers may be programmed to wait for apredetermined amount of time at a given operating condition beforemaking any adjustment. This type of delay could allow the powerconverter to avoid making an unnecessarily large number of adjustments.

It may be advantageous to limit the range of possible adjustments toonly those values that allow the power converter to remain withinspecified operating requirements during any operating conditionspecified in a requirements document. It may also be advantageous tolimit the range of possible adjustments to only those values that ensurethat the components of the power converter do not exceed maximum stresslevels, thereby improving reliability and reducing component or powerconverter failures. For example, a requirements document for a powerconverter may specify operation under a number of transient conditions,such as output load transients, input transients, brown-out conditions,line drop-out conditions, temperature transients, etc.

Turning now to FIG. 10, illustrated is an ac input voltage waveformincluding an exemplary input line voltage dropout transient, showingtime histories of possible internal bus voltages in accordance with theprinciples of the present invention. The FIGURE shows time histories ofpossible internal bus voltages, and an ac input voltage waveform with adrop-out period 1003 during which no ac input voltage is present.Illustrated for the internal bus voltages, is a portion in which theslope 1001 of the internal bus voltage is load dependent. Alsoillustrated in the FIGURE is a bus undervoltage limit 1002. Powerconverters are often required to continue to provide output power for aperiod of time with the ac input voltage at or near zero. This time istypically referred to as the holdup time. When the input line voltagedrops out, the dc-to-dc power converter section (see, e.g., FIG. 3) willcontinue to operate, pulling energy from the holdup capacitors, therebyreducing the voltage on the bus (Vbus). The bus voltage will continue tofall until the line voltage is restored. Note that the slope of the busvoltage will be steeper at a higher output load current. If the busvoltage is allowed to reduce below an under voltage limit, the dc-to-dcpower converter will not be able to support the load and maintainregulation, thereby resulting in an out-of-specification condition. Ifthe exemplary power converter of FIG. 10 is operating at bus voltage“1,” the power converter can operate within specification, but may beoperating at a lower efficiency than desired. If, however, the powerconverter adjusted its bus voltage to bus voltage “2” in an effort toimprove efficiency, the bus voltage will dip below the undervoltagelimit before the end of the drop-out period.

A power converter constructed according to the principles of the presentinvention may sense a variety of input/output operating parameters, andcould thus calculate, for example, the minimum (or a safe) bus voltagethat could both improve efficiency and ensure that the power convertercan maintain the proper holdup time through a line dropout event. Thisis illustrated by bus voltage 3” in FIG. 10. For a given output loadcondition, adjusting the bus voltage to “bus voltage 3” both improvesefficiency and ensures compliance with the specification. Thus, theexemplary power converter is capable of using a multidimensional datatable(s) or other functional representation(s), in conjunction withsensed operating parameters, to determine an operating point withimproved efficiency that also allows the power converter to maintaincompliance with a specification.

There are many examples where adjustments to improve efficiency whilemaintaining compliance with a specification will require a powerconverter to make intelligent adjustments, possibly combining datastored in a multidimensional data table(s) or other functionalrepresentation(s) with sensed operating parameters in the adjustmentcomputation. One such example concerns switching frequency adjustments.It may be advantageous to reduce a switching frequency under, forexample, lighter output load conditions. However, if the load were tosuddenly increase, the power converter controller must ensure that themagnetic components will not be detrimentally affected (by possiblysaturating) at the combination of a higher load condition and a lowerfrequency operating condition, prior to the controller adjusting theswitching frequency to a level more appropriate with the new loadcondition.

Another example can be found in switch timing adjustments, illustratedin FIG. 9F. Such improved switch timing is often dependant on input oroutput current levels. For example, switch timing to improve efficiencyat lighter load may result in cross conduction at heavier loads (or viceversa), thereby causing detrimental operation and possible failure ofthe power converter.

Thus, a controller for a power converter that provides improved powerconversion efficiency has been introduced. Those skilled in the artshould understand that the previously described embodiments of acontroller for a power converter and related methods are submitted forillustrative purposes only. Those skilled in the art understand furtherthat various changes, substitutions, and alterations can be made to thecontroller without departing from the spirit and scope of the inventionin its broadest form. In addition, other embodiments capable ofproviding the advantages as described hereinabove are well within thebroad scope of the present invention. While the controller and methodhave been described as providing advantages in the environment of apower converter, other applications therefor such as a controller for amotor or other electromechanical device are well within the broad scopeof the present invention.

For a better understanding of power electronics, see “Principles ofPower Electronics,” by J. G. Kassakian, M. F. Schlecht and G. C.Verghese, Addison-Wesley (1991). For a better understanding ofsemiconductor devices and processes, see “Fundamentals of III-VDevices,” by William Liu, John Wiley and Sons, (1999). For a betterunderstanding of gallium arsenide processing, see “Modern GaAsProcessing Methods,” by Ralph Williams, Artech House, Second Ed. (1990).The aforementioned references are incorporated herein by reference.

Also, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, many of the controllers discussed above can be implementedin different methodologies and replaced by other processes, or acombination thereof, to form the devices providing improved efficiencyfor a power converter as described herein.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A power converter including a power switch configured to conduct fora duty cycle and provide a regulated output characteristic at an outputthereof, comprising: a controller configured to provide a signal tocontrol said duty cycle of said power switch as a function of saidoutput characteristic, said controller regulating an internal operatingcharacteristic of said power converter to improve an operatingefficiency of said power converter depending on a value of one of aparameter of said power converter measured after a manufacturing stepand an environmental parameter for said power converter.
 2. The powerconverter as recited in claim 1 wherein said controller regulates aninternal operating characteristic of said power converter in accordancewith a table with entries dependent on a value of one of said parameterof said power converter measured after said manufacturing step and saidenvironmental parameter for said power converter.
 3. The power converteras recited in claim 2 wherein said internal operating characteristic isone of an internal bus voltage, a voltage level of a drive signal for apower switch, a number of paralleled power switches selectively enabledto conduct, a temperature of a component part, or a switching frequencyof said power converter.
 4. The power converter as recited in claim 1wherein said controller regulates an internal operating characteristicof said power converter in accordance with a functional relationshipdependent on a value of one of said parameter of said power convertermeasured after said manufacturing step and said environmental parameterfor said power converter.
 5. The power converter as recited in claim 1wherein said environmental parameter is received as a signal from anexternal source.
 6. The power converter as recited in claim 1 whereinsaid internal operating characteristic is regulated by said controlleron a time scale substantially different from a time scale forcontrolling said duty cycle of said power switch.
 7. The power converteras recited in claim 1 wherein said parameter of said power convertermeasured after a manufacturing step is automatically measured in a testfixture.
 8. The power converter as recited in claim 1 wherein saidparameter of said power converter measured after a manufacturing step isa parameter measured from a representative power converter.
 9. The powerconverter as recited in claim 1 wherein said controller regulating aninternal operating characteristic of said power converter to improve anoperating efficiency controls an internal operating characteristic ofsaid power converter in a step-by-step manner during an efficiencyoptimization process.
 10. The power converter as recited in claim 9wherein said controller regulating an internal operating characteristicof said power converter to improve an operating efficiency controls aninternal operating characteristic of said power converter in astep-by-step manner during an efficiency optimization process on a timescale substantially different from a time scale for controlling saidduty cycle of said power converter.
 11. A method of controlling a powerconverter including a power switch configured to conduct for a dutycycle and provide a regulated output characteristic at an outputthereof, comprising: providing a signal to control said duty cycle ofsaid power switch as a function of said output characteristic, therebyregulating an internal operating characteristic of said power converterto improve an operating efficiency of said power converter depending ona value of one of a parameter of said power converter measured after amanufacturing step and an environmental parameter for said powerconverter.
 12. The method as recited in claim 11 wherein said methodregulates an internal operating characteristic of said power converterin accordance with a table with entries dependent on a value of one ofsaid parameter of said power converter measured after said manufacturingstep and said environmental parameter for said power converter.
 13. Themethod as recited in claim 12 wherein said internal operatingcharacteristic is one of an internal bus voltage, a voltage level of adrive signal for a power switch, a number of paralleled power switchesselectively enabled to conduct, a temperature of a component part, or aswitching frequency of said power converter.
 14. The method as recitedin claim 11 wherein said method regulates an internal operatingcharacteristic of said power converter in accordance with a functionalrelationship dependent on a value of one of said parameter of said powerconverter measured after said manufacturing step and said environmentalparameter for said power converter.
 15. The method as recited in claim11 wherein said environmental parameter is received as a signal from anexternal source.
 16. The method as recited in claim 11 wherein saidinternal operating characteristic is regulated on a time scalesubstantially different from a time scale for controlling said dutycycle of said power switch.
 17. The method as recited in claim 11wherein said parameter of said power converter measured after amanufacturing step is automatically measured in a test fixture.
 18. Themethod as recited in claim 11 wherein said parameter of said powerconverter measured after a manufacturing step is a parameter measuredfrom a representative power converter.
 19. The method as recited inclaim 11 wherein said method regulates an internal operatingcharacteristic of said power converter to improve an operatingefficiency controls an internal operating characteristic of said powerconverter in a step-by-step manner during an efficiency optimizationprocess.
 20. The method as recited in claim 19 wherein said methodregulates an internal operating characteristic of said power converterto improve an operating efficiency controls an internal operatingcharacteristic of said power converter in a step-by-step manner duringan efficiency optimization process on a time scale substantiallydifferent from a time scale for controlling said duty cycle of saidpower converter.